Integrated optical grating device by thermal SiO2 growth on Si

ABSTRACT

Disclosed is a method of producing an integrated optical grating device having extremely low scattering losses. The method employs a highly polished and prepared silicon chip for receiving a grating pattern or any other surface relief feature on the silicon wafer surface for its predetermined use. The pattern after it is generated is etched to a predetermined period, for example, when the pattern is for a waveguide device. An SiO 2  growth layer is thermally grown to a thickness of about 4 to 8 micrometers to replicate the generated pattern in the SiO 2  growth layer. This method yields a waveguide or similar optical grating device having undulations of extremely low values as determined by SEM photographs of this device.

DEDICATORY CLAUSE

The invention described herein was made in the course of or under acontract or subcontract thereunder with the Government; therefore, theinvention described herein may be manufactured, used and licensed by orfor the government for governmental purposes without the payment to meof any royalties thereon.

BACKGROUND OF THE INVENTION

Prior art has taught the use of gratings in SiO₂ on Si integratedoptical structures to deflect the guided optical waves, focus them, andconfine waves to desired optical circuit regions. Also, prior art hastaught the fabrication of these gratings by holographic exposure ofphotoresist layers on the integrated optic surface and sometimessubsequent etching of the waveguide or other optical layers on top ofthe waveguide or below the waveguide to generate the guided wave-gratinginteraction.

The development program of a low loss optical waveguide by using thenatural oxide of a polished silicon wafer as the beginning material hasevolved along two routes or two stages. The first stage of developmentpursued the route directed to the determination of SiO₂ formation rateson polished Si wafers, and the effects encountered as long,high-temperature processing is performed. The second stage ofdevelopment pursued the route directed to the treatment of the SiO₂layer in order to increase the upper film refractive index and toincrease the wave binding to the top of the film and avoid theabsorption incurred when the evanescent tail of the bound mode extendsinto the Si substrate.

The growth of SiO₂ substrate is carried out by holding the Si wafertemperature between 900°-1050° C. in steam or O₂ at atmosphericpressure. The oxide grown is roughly twice the thickness of the Simaterial used. The growth rate of the steam oxide system indicates thatan oxide thickness in micrometers (μm) of 2, 3, 7, and 12 require growthtimes of 10, 30, 100, and 300 hours at 1 atmosphere H₂ O. This system ofgrowth is for (100) oriented Si with slight (10¹⁴) boron doping. Thisgrowth rate is greatly enhanced by increasing the steam pressure. At 10times atmospheric pressure, the oxide formation rate is about five timesthe one atmosphere rate. The growth rate of SiO₂ is dependent on all theparameters of temperature pressure, orientation, resistivity, and soforth as taught by C. P. Ho and J. D. Plummer: Journal ofElectrochemical Society, Vol. 126, No. 9, pp. 1516-1530.

The effect of creating a thick oxide of 5-15 μm on a Si wafer introducestremendous strains at the interface of the film when the oxidized waferis cooled to room temperature. This effect is evaluated by reflectiontechniques wherein a three-inch-diameter wafer with a 14 μm oxide isemployed to reflect the image of a white ruler. The strong curvature isevident. This curvature is caused by the different oxide thickness onthe polished front versus the unpolished (fine ground) reverse side ofthe wafer. For oxide thicknesses of less than 8 μm, this deformation hasnot been observed.

Other deformations of the Si wafer are observed with all the oxidethicknesses, however; and they are even more difficult to eliminate andare certainly more important to the future generation of low losswaveguides on Si subtrates. The crystalline imperfections, dislocations,etc., will propagate when the temperature is high, and the stresses dueto the oxide may contribute to this process. These imperfections movefrom the outer region of the crystal where the growth process hasproduced them, to the center region of the wafer. This causes the centerregion (˜1" diam) of the wafer to appear nonuniform and undulated, andthe inclination to spontaneous cleavage in undesirable places is adanger to the wafer. This effect is less of a problem when the substrateresistivity is low (<1Ω-cm).

The as-grown thick SiO₂ has built-in wave binding condition such thatthe undoped oxide is a low-scattering, relatively low-loss (˜3 dB/cm)waveguide. This factor has not yet been explained but the cause of thisinherent waveguide is suspected to be related to the thermal expansioncoefficient difference between Si and SiO₂ and the large stressesintroduced in the film. Also, the SiO₂ density varies with thickness andthis contributes to the index profile. The mode index and mode depthversus oxide thickness has been evaluated for the steam oxidewaveguides. Clearly the refractive index of the thermally grown SiO₂does not vary a lot since the TE₀ mode index is about 0.008 greater thanthe value for thin (<1 μm) thermal oxide SiO₂ layers, and the mode depthis a significant fraction of the SiO₂ film thickness. Also, the linearrelationship between mode thickness and SiO₂ total thickness suggeststhe mechanism generating the index differential is held near theSi--SiO₂ interface.

Most waveguides have surface scattering as the dominate loss mechanism;therefore, if the dominate cause of loss can be identified andcorrective procedure applied, a low-scattering waveguide may bedeveloped to have an ultra-low loss. The loss is observed to decreasewith increasing oxide thickness where more energy propagates in thelow-loss SiO₂ further away from the absorbing Si substrate. Thisconclusion is reasoned from the observed increase of mode depth withincreasing oxide thickness. Thus the preceding argument indicates thatthe dominate mechanism for loss in the thick oxide waveguides is siliconabsorption.

It is clear, therefore, that the thermal oxide waveguide is somewhatunsatisfactory, or at least not an optimum selection to the need for alow-loss waveguide. The second stage of the waveguide developmentprocess was pursued with the goal of treating the oxide surface in sucha way as to increase the refractive index near the surface and bind thewave more closely to upper surface away from the Si substrate.

Techniques applied by others to form optical waveguides were consideredin the plan to reduce the loss in the thick SiO₂ waveguides. Facilitiesemployed such techniques as sputtering and evaporation coatingequipment, high temperature diffusion furnaces and ion implantation to500 kV. Materials employed as surface treatment components includedphosphorous, boron, 7059 glass, lead oxide, copper oxide, alumina, andtitanium oxide. The implanter was used to dope the SiO₂ with boron andphosphorous, and the other materials were used as diffusant sources inthin film layers on the thermal oxide film.

Efforts pursued by the described techniques have not achieved thedesired ultra-low loss optical waveguide. Because of the imperfectionsof the Si substrate (crystalline imperfections, dislocations, etc.,which will propagate when the temperature is high and the stresses dueto different oxide thickness on the polished front versus theunpolished, fine ground, reverse side of the wafer), and because of thedifficulty in handling the oxidized wafers and, in particular, thisdifficulty which relates to the fragile nature of the SiO₂ surface, animproved method is desired for producing more perfect gratings in awaveguide requiring low scattering losses. This desired method isenhanced by failure of two techniques pursued wherein the grating isrequired to be coupled to the waveguide mode either as spatialmodulation of the waveguide upper or lower boundary, or as a filmdeposited upon the upper boundary. Although both techniques werepursued, with the primary effort being devoted to the corregated surfacegratings, the thin film gratings of photoresist did not provide the lowscattering grating required for a desired device.

Gratings established in SiO₂ waveguide by ion milled technique with astep height of the grating of about 1000 Å proved to be unsuitable forefficient beam reflection due to the nature of the edge of the strip.The roughness of the edge caused intense scattering of the guided wavebeam out of the waveguide. Some mode coupling is expected due to thethickness step, but the roughness of several microns is far too coarsefor low loss devices.

Therefore, an object of this invention is to provide a method of growinga waveguide of SiO₂ wherein the SiO₂ is produced as a replica of anoriginal Si surface.

A further object of this invention is to produce an integrated opticalgrating device by a method wherein a grating pattern is firstholographically produced on an original highly polished, Si surface, theSi surface pattern is milled away by reactive ion etching, chemicaletching, or ion beam milling, and a SiO₂ formation process is completedto grow a 4-8 μm oxide layer which produces the grating replicated inSiO₂.

SUMMARY OF THE INVENTION

An integrated optical grating device is fabricated in a polished silicon(Si) wafer surface by holographic exposure-etching techniques. A gratingor the desired surface relief feature is generated in the polished Siwafer surface, and the SiO₂ is grown on top of the Si wafer surface,thereby achieving a SiO₂ replica of the grating or surface relieffeature generated in the polished Si wafer.

The prepared Si wafer, having the grating or desired surface relieffeature generated thereon, is heated to about 1000° C. in H₂ O vapor at1 to 10 times atmospheric pressure to thereby produce a thick SiO₂ layerwhich contains the waveguide and grating device simultaneously. Thethick layer (≈4-8 μm) of SiO₂ as Si has extremely low scattering losses.The method of this invention allows the generation of the gratings orany other surface relief features on the Si wafer surface prior to thewaveguide growth. The desired surface relief features are easier toproduce on the Si surface and the SiO₂ grown thereon to produce thereplica of the relief features rather than producing the features on theSiO₂ surface by reactive ion etching techniques. The resulting waveguidecontains the desired grating or other surface structure without havingcompromised the low scattering properties of the waveguide.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 of the drawing depicts a grating pattern generated in a highlypolished silicon wafer surface, and

FIG. 2 depicts the grating pattern replicated in a SiO₂ growth layergrown on the highly polished silicon wafer surface generated in thesilicon wafer surface.

FIG. 3 depicts an enlarged sectional view, partially cut-away, of awaveguide having a grating pattern replicated in a SiO₂ growth layer.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The method of this invention produces an integrated optical gratingdevice by thermal SiO₂ growth on a polished and prepared silicon waferhaving a grating or surface relief pattern on the silicon wafer surface.

The method comprises providing a highly polished silicon (Si) waferwhich serves as the substrate or a chip on which a waveguide isconstructed, holographically producing a grating on the Si wafersurface, milling away a portion of the Si wafer surface by reactive ionetching, chemical etching, or ion beam milling, and completing a SiO₂formation process to grow a 4-8 μm growth of SiO₂ on the Si chip surfacewherein the grating is replicated in the SiO₂.

In further reference to the figures of the drawing FIG. 1 depicts asilicon wafer 10 having a highly polished surface 12 with gratingpattern 14, ion milled into the silicon surface. FIG. 2 depicts asilicon wafer 20 with a 5.5 micrometers steam growth layer of SiO₂ 16and further showing the grating 14 of FIG. 1 as it appears in the formof the replicated grating 18 in the SiO₂ surface layer. FIG. 3 depictsan enlarged, sectional view of a waveguide device 20 which depicts asilicon chip 10 having a highly polished silicon surface 12 that has agrating pattern or surface relief feature 14 which is generated as areplica 18 of the grating or surface relief feature in the SiO₂ growthlayer 16.

The SiO₂ formation process employed to replicate the grating patterngenerated in the silicon surface is described in detail hereinbelow.

The SiO₂ formation process comprises heating the Si wafer, having agrating pattern prior etched in the Si wafer surface, to 1000° C. in H₂O vapor of 1 to 10 times atmosphere pressure for a predetermined timeperiod to grow a SiO₂ layer from about 4-8 μm thickness on the Si wafersurface to thereby produce the grating in the waveguide by growing theSiO₂ layer as a replica of the original Si surface.

A growth time from about 2 to about 60 hours is required where the watervapor pressure is maintained at about 10 atmospheres and a temperatureof about 1000° C. to grow a SiO₂ layer over the range from about 2 to a12 micrometers thickness. The growth time is increased to about 10 toabout 300 hours for a 1 atmosphere pressure and like thickness grown atthe higher atmosphere pressure.

The generation of the gratings or any other surface relief features onthe Si wafer surface prior to the waveguide growth permits theprefabrication of all elements on the Si surface where surface relief iseasier to produce than on the SiO₂ surface by reactive ion etch. Theresulting waveguide contains the desired grating or other surfacestructure without having compromised the low scattering properties ofthe waveguide.

SEM photograph of a 2500 Å period grating as ion beam milled into a Sisurface indicated about 300-500 Å undulations with a 2500 Å period;however, an SEM photograph of the same grating after a 5.5 μm growth ofSiO₂ indicating a smoothing out of this profile. Several demonstrationsof patterns produced by reactive ion etching have shown that a gratingof 2500 Å period and 1 μm undulation can be achieved. This amount ofmilling would probably not be required, but could be available tocontrol the top surface undulation depth. The SEM photographs distinctlyshow the grating structure in the surface even though the period of thegrating is a factor of 22 smaller than the SiO₂ growth of 5.5 μm.

FIGS. 1 and 2 of the drawing illustrate the features shown verydistinctly by SEM photographs as described hereinabove.

I claim:
 1. A method of producing an integrated optical grating devicehaving extremely low scattering losses from an optical grating patternor other surface relief feature generated in a highly polished andprepared silicon wafer surface and replicated in a subsequently grownthermal layer of SiO₂, said method comprising:(i) providing a siliconwafer having a highly polished surface and prepared for receiving agrating pattern or any other surface relief feature pattern on saidsilicon wafer surface for its predetermined use; (ii) generating saidgrating pattern or other surface relief feature pattern on said highlypolished silicon wafer surface by a holographic exposure-etchingtechnique; (iii) milling away a portion of the silicon wafer surface byreactive ion etching, chemical etching, or ion milling to produce saidgrating or other surface relief feature in accordance with said patterngenerated on said highly polished silicon wafer surface; and, (iv)growing a 4-8 micrometers thick growth layer of SiO₂ on said siliconwafer wherein said produced grating or other surface relief feature isreplicated in said SiO₂ growth layer.
 2. The method of producing anintegrated optical grating device as disclosed by claim 1 wherein saidgrowing of 4-8 micrometers thick growth layer of SiO₂ on said siliconwafer having said generated pattern or other surface relief feature iseffected by a SiO₂ formation process which comprises heating saidsilicon wafer to about 1000° C. in water vapor of 1 to 10 timesatmospheric pressure for a predetermined time period to effect thegrowth of a SiO₂ layer to said thickness.
 3. The method of producing anintegrated optical grating device as disclosed by claim 2 wherein saidSiO₂ formation process employs a predetermined growth time from about 10to about 300 hours and wherein said water vapor is at about 1 atmosphereof pressure.
 4. The method of producing an integrated optical gratingdevice as disclosed by claim 2 wherein said SiO₂ formation processemploys a predetermined growth time from about 2 to 60 hours and whereinsaid water vapor is about 10 atmospheres.
 5. The method of producing anintegrated optical grating device as disclosed by claim 4 wherein saidgrating produced in said highly polished silicon wafer surface has aperiod of about 0.25 micrometer, and wherein said SiO₂ growth layer isabout 5.5 micrometers thick which replicates said grating in the topsurface of said SiO₂ growth layer, said replicated grating in said SiO₂growth layer additionally characterized by having an extremely lowscattering loss when functioning as a waveguide or similar functioningdevice.